One of the problems encountered in implementing optical sensors is that no satisfactory scheme has been yet devised for multiplexing passive optical sensors onto a single fiber-optic bus. The techniques that have been proposed and tried to date include an optical time domain reflectometer bus, and wavelength division multiplexing. In the optical time domain reflectometer bus, the source is pulsed, and each sensor responds to the pulse. Because the sensors are separated spatially along the bus, the responses will be received by the detector as a time multiplexed data stream. A problem associated with this method of multiplexing is that it is difficult to fabricate suitable sensors.
In the wavelength division multiplexing approach, the wavelength of the source is ramped, the source is broadband in nature, or the outputs of a number of sources of different wavelengths are combined, and each sensor responds to a specific wavelength. The problem with this technique is that it is difficult to find suitable broadband sources, or a source that can be ramped over an adequate wavelength range. A further problem is that the available choices of sensors that respond at different wavelengths is quite limited.
A technique termed coherence multiplexing has recently been devised for multiplexing optical signals onto a single bus. This technique may be explained with reference to FIG. 1, which illustrates a prior art single sensor (nonmultiplexed) system comprising laser diode 12, sensor 14 and detector 16. Sensor 14 consists of a Mach-Zehnder interferometer comprising couplers 20 and 22 and fiber-optic cables 24 and 26 that comprise the two arms of the interferometer. Electromagnetic radiation from laser diode 12 is coupled through fiber-optic cable 30 to coupler 20, and coupler 20 divides the radiation between fiber-optic cables 24 and 26. Radiation exiting from the opposite ends of the fiber-optic cables is combined by coupler 22 onto output fiber-optic cable 32.
Sensor 14 includes means for modulating the optical path length of one of arms 24 or 26, for example arm 26, in accordance with a sensed input parameter. Known fiber-optic sensors of this type include electric and magnetic field sensors, hydrophones, and temperature sensors. However for the system of FIG. 1, the optical path length difference between arms 24 and 26 is selected such that for all expected values of the sensed parameter, the path length difference between the arms is greater than the coherence length of laser diode 12. As a result, a change in the relative phase between the arms of the interferometer will not be converted into a detectable intensity modulation at the interferometer output on fiber-optic cable 32. Nevertheless, the phase information generated by sensor 14 will be retained.
Detector 16 comprises photodetector 40, and a Mach-Zehnder interferometer comprising couplers 42 and 44 and fiber-optic cable arms 46 and 48. The signal provided by sensor 14 on fiber-optic cable 32 is input to coupler 42, and split by the coupler between arms 46 and 48. The signals exiting from the opposite ends of the arms are combined by coupler 44 and conveyed by fiber-optic cable 50 to photodetector 40. Detector 16 is designed such that the optical path length difference bertween arms 46 and 48 differs from the optical path length difference between arms 24 and 26 by an amount less than the coherence length of laser diode 12, for all expected values of the sensed parameter. As a result, the difference in optical path length between a composite sensing path comprising arms 26 and 46, and a composite reference path comprising arms 24 and 48, is made less than the coherence length of source 12 for all expected values of the sensed parameter. Thus when the radiation that has traveled through the composite sensing and reference paths is combined by coupler 44 onto fiber-optic cable 50, interference will be produced. Therefore, as the optical path length of arm 26 is modified by the sensed input parameter, a detectable modulation of the signal on fiber-optic cable 50 will be detected by photodetector 40.
A typical output produced by detector 16 is illustrated in FIG. 2, which shows the relative intensity of the radiation propagating along fiber-optic cable 50 as the optical path length of arm 26 is varied by variation of the sensed parameter. The intensity includes a steady, DC level that results from radiation that has passed through composite paths comprising arms 24-46 and 26-48, and a superimposed interference pattern resulting from radiation that has passed through the composite sensing path and the composite reference path. At point 52, the composite sensing and reference path lengths are equal, and the radiation that has traversed such paths interferes constructively to produce a maximum in the output intensity. As the path length of arm 26 varies in either direction from point 52, fringes of diminishing intensity are produced. As the difference between the composite sensing and reference paths approaches and exceeds the coherence length of source 12, the fringe amplitude decreases to zero, and no amplitude variations are produced.
The extension of the above-described concepts to a multiplexed multisensor system has been proposed. In such a system, each sensor/detector pair would have a path length difference that was larger than the coherence length of the source laser diode, and that was also different from the path length differences of the other sensor/detector pairs by an amount greater than the coherence length of the source. As a result, each detector output would be modulated only by path length changes introduced by its corresponding sensor. However, the difficulty with such a multiplexing technique is that it is difficult to find illumination sources having appropriate coherence lengths. The coherence length of a laser diode is of the order of meters. Thus in a laser diode system, the fiber-optic cable arms of each interferometer must have a path length difference on the order of ten or more meters, and its associated detector must have a similar path length difference. It would be extremely cumbersome to implement a multisensor system having fiber-optic cables of such lengths. The use of optical sources having much shorter coherence lengths has also been proposed. For example, an LED or superluminescent diode (SLD) has a coherence length on the order of 15 microns. There are advantages in using a source having a short coherence, such as reduced phase noise. However, a severe practical problem in using an SLD source would be that of controlling the path lengths of fiber-optic cables to accuracies on the order of tens of microns.